Lithium secondary batteries have received attention as high-capacity power sources for portable and other appliances. Further, lithium secondary batteries have recently been receiving attention as high-output power sources for electric vehicles and the like. Chemical batteries such as lithium secondary batteries usually have a separator that electrically insulates a positive electrode from a negative electrode and holds an electrolyte. In the case of a lithium secondary battery, a micro-porous film made of polyolefin (e.g., polyethylene, polypropylene, etc.) is mainly used as the separator. The electrode assembly of a prismatic lithium secondary battery is produced by winding the positive electrode, the negative electrode, and the separator interposed between the two electrodes such that the wound assembly is substantially elliptic in cross-section.
However, when a lithium secondary battery is stored in an environment at extremely high temperatures for an extended period of time, its separator made of a micro-porous film tends to shrink. If the separator shrinks, then the positive electrode and the negative electrode may physically come into contact with each other to cause an internal short-circuit. In view of the recent tendency of separators becoming thinner with an increase in lithium secondary battery capacity, preventing an internal short-circuit becomes particularly important. Once an internal short-circuit occurs, the short-circuit may expand due to Joule's heat generated by the short-circuit current, thereby resulting in overheating of the battery.
Thus, in the event of an internal short-circuit, in order to suppress such expansion of the short-circuit, Japanese Laid-Open Patent Publication No. Hei 7-220759 proposes forming a porous heat-resistant layer that contains an inorganic filler (solid fine particles) and a binder on an electrode active material layer. Alumina, silica, or the like is used as the inorganic filler. The inorganic filler is filled in the porous heat-resistant layer, and the filler particles are bonded to one another with a relatively small amount of a binder. Since the porous heat-resistant layer is resistant to shrinking even at high temperatures, it has the function of suppressing the overheating of the battery in the event of an internal short-circuit.
Recently, in the field of the power source for portable appliances, there is an increasing need for rapid charge, and rapid charge requires charging at a high rate (e.g., 1 hour-rate or less). In the case of a high-rate charge, the electrode plate expands and contracts significantly during charge/discharge and a large amount of gas is produced, compared with a low-rate charge (e.g., 1.5 hour-rate or more). Therefore, the electrode assembly is distorted, and the porous heat-resistant layer may break since the amount of the binder contained in the porous heat-resistant layer is relatively small and the bonding between filler particles is weak. In such cases, the function of the porous heat-resistant layer of suppressing the overheating of the battery in the event of an internal short-circuit is impaired.